daysbetween function

I want to calculate the amount of days left between now and let's say 5 days from now using daysbetween. everytime i try to run my app, it says that daysbetween is a an undeclared identifier. i want to use this function to limit the length of time a client can use my application. basically a trial version the can try out for a couple of days before it locks up and require serial key to reactivate. usually made a monthly trial but want to go to weekly one. can anybody give me some tips on this.
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